@Ernir covers this well.
The question is whether or not a maneuvers that you know are separated by the classes you are in or not. In other words, does a Crusader/Warblade have separate Crusader maneuvers and Warblade maneuvers, or just a single list of maneuvers? The book is not entirely clear.
Certainly, the Crusader and Warblade classes get access to some different disciplines (Devoted Spirit for Crusader; Diamond Mind, Iron Heart, and Tiger Claw for Warblade), but those restrictions are on which maneuvers they can learn as a part of leveling in those classes, not the maneuvers they can know. After all, they can use Martial Study or prestige class levels to get maneuvers from other disciplines.
And they definitely maintain separate lists of readied maneuvers, which affect how each maneuver is recovered and how many maneuvers can be readied at once. The book wouldn’t make any sense otherwise.
But the question remains whether or not the lists of maneuvers known are kept separated.
If they are not, then you cannot ever ready a maneuver more than once, because once you know the maneuver you know it and that’s the end of it. However, that also means that you can ready maneuvers that you “learned” from one class as a maneuver from the other class (assuming you have a high enough Initiator Level in the second class). For instance, a Crusader 8/Warblade 1 could learn iron heart surge (a 3rd-level Iron Heart maneuver, requires IL 5) with that Warblade level (½×8+1=5), but then ready it as one of his Crusader maneuvers (taking advantage of the Crusader’s generally-superior recovery mechanic).
If the lists are separated, however, so that you have your Warblade maneuvers known and your Crusader maneuvers known and every maneuver you learn is added to one or the other and cannot be readied from the opposite class, then you could learn the same maneuver with both classes: it only appears on a maneuvers known list once. And then you could ready it with each class, effectively readying it twice.
The book is not clear on this. The most common way to play, that I’ve seen, is actually a logically-inconsistent mix: you can not ready another class’s maneuvers but you can’t relearn or reready a maneuver that you know from another class either, as if the lists were both separate and combined at the same time. This, to me, is not a tenable position, however: the two options are mutually exclusive and half-one-way-half-the-other results in a kind of weird Schrödinger’s maneuver list.